A frequently employed and more or less standardized platinum temperature sensor has the type designation PT-100 and this has a primary interest in this connection. Known temperature measurement circuits, for co-operation with such platinum resistors, are available in the form of an integrated circuit or chip, among other things based on a three conductor principle at the input side for connection to the platinum resistor, the reference resistor respectively and with a common return lead as mentioned above. As far as the output of the measurement circuit is concerned, the invention is directed to a situation where the output DC current mentioned appears at two output terminals, for simple and practical adaption to the circuits or the system in which the temperature measurement is to be involved.
More particularly there exist previously known circuits which carry out the necessary linearizing in connection with, for example, the plantinum resistor of type PT-100 mentioned, including solutions based on an increase of the applied current in the sensor when the temperature rises.
U.S. Pat. No. 3,783,692 shows a measurement circuit in which a platinum sensing resistor is incorporated in a bridge configuration. Linearizing of the sensor is obtained by increasing the current therein by means of a specific current source having a negative resistance. Here there is the question of a rather complicated circuit in which it is difficult to calculate the component values for adaption to different temperature ranges to be covered by the measurements.
U.S. Pat. No. 3,754,442 describes a measurement circuit adapted to co-operate with different sensors having different non-linearity. The sensor is connected into a feed-back loop in an operational amplifier. A second operational amplifier controls an indicator and acts to vary the current in the sensor depending upon the output voltage. A rather complicated circuit is also involved there, due to among other things, it is to co-operate with different sensors.
European Patent 0 101 956 relates to a measurement circuit designed with a common differential amplifier in cooperation with a traditional source of current in a four-conductor measurement principle. The output voltage controls the source of current so that the current in the sensor increases with a rising temperature.
U.S. Pat. No. 4,114,446, similarly to some of the patent specifications above, is directed to a relatively complicated measurement circuit comprising a source of current and various amplifiers which by means of several feed-back loops are adapted to linearize the sensing resistor.